The present invention, in some embodiments thereof, relates to isolated complexes of MHC class II and glutamic acid decarboxylase (GAD) autoantigenic peptides, isolated high affinity entities such as antibodies which specifically bind to same and, more particularly, but not exclusively, to uses thereof for diagnosing and treating type I diabetes.
Major histocompatibility complex (MHC) class II molecules are expressed in professional antigen presenting cells (APCs) such as macrophages, dendritic cells and B cells. Each MHC class II molecule is a heterodimer composed of two homologous subunits, alpha chain (with α1 and α2 domains) and beta chain (with β1 and β2 domains). Peptides, which are derived from extracellular proteins, enter the cells via endocytosis, are digested in the lysosomes and further bind to MHC class II molecules for presentation on the membrane.
Antigen-specific activation or regulation of CD4+ T cells is a multistep process in which co-ligation of the T cell receptor (TCR) with complexes of MHC II/peptide on the surface of APCs plays a central role.
MHC class II molecules with bound self peptides presented by professional APCs play a central role in activating specific CD4+ T cells involved in autoimmune diseases such as type 1 diabetes (T1D).
T1D (also known as juvenile diabetes) occurs when the autoimmune destruction of pancreatic beta-islet cells prevents production of the hormone insulin. This causes an inability to regulate glucose metabolism, which results in dangerously raised blood glucose concentrations. It is generally accepted that thymus-derived lymphocytes (T cells) are critically involved in the onset and progression of type 1 diabetes, but the antigens that initiate and drive this destructive process remain poorly characterized—although several candidates have been considered such as insulin, insulin derivatives, islet-specific glucose-6-phosphatase catalytic subunit related peptide (IGRP), carboxypeptidase H, insulinoma-associated antigen (IA-2), glutamic acid decarboxylase (GAD65), carboxypeptidase E and heat shock protein 60.
Genetic factors affecting susceptibility to T1D include the insulin-dependent diabetes mellitus 1 (IDDM1) gene (GeneID 7924) which is located in the MHC class II region on chromosome 6p21 and which is likely to be responsible for the histocompatibility disorder characteristic of type 1 diabetes in which pancreatic beta cells display improper antigens to T cells. Linkage analysis shows that 96% of diabetic patients express HLA-DR3 and/or HLA-DR4, including over-representation of the HLA-DR3/DR4 heterozygosity in diabetics as compared with non-diabetic controls. These alleles are tightly linked to HLA-DQ alleles that confer susceptibility to IDDM. Other non-genetic factors which might affect susceptibility to type 1 diabetes include diet, which affects gut flora, intestinal permeability, and immune function in the gut.
Glutamate decarboxylase (GAD) enzyme in mammals exists in two isoforms-GAD 65 kDa (GAD2; GeneID 2572) and GAD 67 kDa (GAD1; GeneID 2571). While both isoforms are expressed in brain, GAD 65 kDa is also expressed in the pancreas. Importance of GAD as an islet autoantigen initially highlighted because of the high frequency of auto-antibodies in patient sera directed against this molecule. Subsequent studies led to a large accumulation of data, which support the notion that a dominant CD4+ T-cell response to GAD 65 kDa is a relevant marker for cellular autoimmunity in T1D (Nepom G T. 2003. Conversations with GAD. J Autoimmun. 20:195-8).
Based on the high association of the HLA-DR4 gene to T1D, many epitope identification studies were done, revealing a limited number of GAD peptides presented by the DR4 molecule (Nepom, G. T., et al., 2001). Human CD4+ T cell responses to the DR4/GAD peptides were obtained both among T1D patients and controls (Masewicz, S. A., et al., 2002; Bach, J. M. et al., 1997; Ou, D., et al., 1999; Roep, B. O., et al., 1999; Lohmann, T. et al., 1996; Rharbaoui, et al., 1999), suggesting that the potential for autoreactivity is present in many individuals.
GAD555-567 peptide in the context of HLA-DR4 has been shown to be an efficiently processed immunodominant epitope in patients with type 1 diabetes and DR401 transgenic mice (Reijonen, H., et al., 2002; Patel, S. D., et al., 1997). DR4/GAD555-567 tetramer detection of autoreactive CD4+ T-cells were observed in the peripheral blood of T1D and at risk subjects but not in healthy controls (Oling, V., et al., 2005).
Additional background art includes U.S. Patent Application Publication No. 20020114816 (ENDL, JOSEF; et al.); U.S. Patent Application Publication No. 20090155292; U.S. Patent Application No. 20030166277; and Krogsgaard M., et al., 2000, Journal of Experimental Medicine, Pages 1395-1412).